An electrical filter is a device designed to separate, pass or suppress a group of signals from a mixture of signals. Filters are basic electronic components used in the design of communication systems such as telephone, television, radar and computers.
As is known in the art, an RF filter circuit provides a relatively low insertion loss characteristic to all RF signals having a frequency corresponding to one of a first predetermined band of frequencies. The first predetermined band of frequencies is generally referred to as a pass-band of the filter circuit. The RF filter further provides a relatively high insertion loss characteristic to RF signals having a frequency corresponding to a second predetermined band of frequencies. This second predetermined band of frequencies is generally referred to as the stop-band of the filter circuit. The filter circuit may be provided having a combination of pass-bands and stop-bands to provide a filter circuit having low-pass, high-pass and band-pass filter characteristics all well known to those of skill in the art.
As is also known in the art, RF filter circuits provided from printed circuit fabrication techniques are preferred because of the low cost and simplicity of the manufacturing process. Printed circuit filters are provided from a plurality of strip conductors disposed on a substrate. Such filter circuits maybe provided for example in a microstrip configuration or in a strip line configuration as is well known in the art. In a particular class of printed circuit filters, referred to as coupled line filters, strip conductors are disposed on the substrate in proximity to one another such that coupling occurs .between adjacent portions of the strip conductors.
The impedance characteristics and the coupling between the strip conductors cooperate to provide RF filter circuits having desired pass-band and stop-band characteristics. Regardless of Whether coupled line filters are provided in microstrip or stripline configurations, the filter circuit generally includes strip conductors having regions with electrical pathlengths corresponding to some fraction of a wavelength at the desired frequency of operation (e.g., one-quarter or one-half wavelength). Moreover, the fractional wavelength coupling regions are disposed to provide a plurality of coupled line sections with each coupled line section of the filter typically having substantially identical length.
The filter characteristics ( i. e., insertion loss, bandwidth, pass-band, and slope of the .so-called filter skirts) are, among other things, directly related to the number of poles provided in the filter. For example, in the case of parallel coupled line filter, the number of poles is proportional to the number of coupling sections. Many coupling sections are needed to provide a filter having a narrow pass-band, sharp filter skirts and a stopband having a high insertion loss. Thus, a filter providing the aforementioned electrical characteristics will be a relatively large circuit. This is a major disadvantage since size and weight are important factors for today's electronic hardware.
Thus, what is needed is an improved filter that rejects unwanted signals close to the pass-band, and provides a narrow pass-band, sharp filter skirts and a stop-band having a high insertion loss, accomplished with fewer poles or coupling sections to maintain a compact size and low weight. What is further needed is method of rejecting unwanted signals close to the pass-band, and providing a narrow pass-band, sharp filter skirts and a stop-band having a high insertion loss, accomplished with fewer poles to maintain a compact size and low weight.